1. Field of the Invention
The present invention relates to oil well drilling tools. In particular, the present invention relates to nuclear magnetic resonance measurement while drilling tools.
2. Description of the Related Art
In the oil and gas industry, hydrocarbons are recovered from formations containing oil and gas by drilling a borehole to the formation using a drilling system. The system typically comprises a drill bit carried at an end of a drill string. The drill string is comprised of a tubing which may be drill pipe made of jointed sections or a continuous coiled tubing and a drilling assembly that has a drill bit at its bottom end. The drilling assembly is attached to the bottom end of the tubing. To drill a well bore, a mud motor carried by the drilling assembly rotates the drill bit, or the bit is coupled to drill pipe, which is rotated by surface motors. A drilling fluid, also referred to as "mud," is pumped under pressure from a source at the surface (mud pit) through the tubing. The mud serves a variety of purposes. It is designed to provide the hydrostatic pressure that is greater than the formation pressure to avoid blowouts. The mud drives the drilling motor (when used) and it also provides lubrication to various elements of the drill string.
The mud flowing in the annular space between the drill string and borehole wall will invade the formation for a short distance due to the pressure exerted by the mud. Although this increased pressure helps prevent blowouts, the region, known as the invasion or invaded zone, becomes contaminated with the mud. Any measurements on formation fluids within the invasion zone may be inaccurate because of the contamination. For this reason, it is desirable to sample and test fluids beyond the invaded zone.
During drilling operations, information on a specific formation is gathered once the borehole reaches the area known as the zone of interest. Downhole instruments and/or sampling devices are utilized at the zone of interest to gather data regarding various parameters of interest including pressure, temperature and other physical and chemical properties of the formation fluid and or mud. These data-gathering operations during drilling are known as measurement while drilling (MWD) or logging while drilling (LWD). The differences between MWD and LWD are known in the art and are not particularly relevant to the present invention. Therefore, the focus will be limited to LWD terminology. The intent, however, is to include MWD, along with wireline logging, embodiments and methods within the scope of the present invention.
One LWD method used to determine characteristics of formation fluid is known as nuclear magnetic resonance or NMR well logging. NMR well logging instruments can be used for determining properties of earth formations including the fractional volume of pore space and the fractional volume of mobile fluid filling the pore spaces of the earth formations. Methods of using NMR measurements for determining the fractional volume of pore space and the fractional volume of mobile fluid are described, for example, in Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination, M. N. Miller et al, Society of Petroleum Engineers paper no. 20561, Richardson, Tex., 1990.
An NMR log is dependent on the alignment of the magnetic moment of protons with an impressed magnetic field. In NMR logging applications, the protons of hydrogen nuclei are of interest. The spins of protons tend to align themselves with the magnetic field. NMR instruments typically make measurements corresponding to an amount of time for hydrogen nuclei present in the earth formations to substantially realign their spin axes, and consequently their bulk magnetization, with an applied magnetic field. The applied magnetic field is typically provided by a permanent magnet disposed in the instrument. The spin axes of hydrogen nuclei in the earth formation, in the aggregate, align with the magnetic field applied by the magnet.
The NMR instrument also typically includes an antenna, positioned near the magnet and shaped so that a pulse of RF power conducted through the antenna induces an RF magnetic field in the earth formation. The RF magnetic field is generally orthogonal (perpendicular) to the field applied by the magnet. The first RF pulse, typically called a 90-degree pulse, has a duration and amplitude predetermined so that the spin axes of the hydrogen nuclei generally align themselves perpendicularly to the static magnetic field applied by the magnet. After the 90 degree pulse ends, the nuclear magnetic moments of the hydrogen nuclei gradually "relax" or return in a precessional rotation to their original alignment with the field of the magnet. The amount of time taken for this relaxation is related to petrophysical properties of interest of the earth formation.
The precessional rotation generates RF energy at a frequency proportional to the strength of the magnetic field applied by the magnet, this frequency being referred to as the Larmor frequency. The constant of proportionality for the Larmor frequency is known as the gyromagnetic ratio (.gamma..sub.0). The gyromagnetic ratio is unique for each different chemical elemental isotope. The spin axes of the hydrogen nuclei gradually "dephase" because of inhomogeneities in the magnet's field and because of differences in the chemical and magnetic environment within the earth formation. Dephasing results in a rapid decrease in the magnitude of the voltages induced in the antenna. The rapid decrease in the induced voltage is referred to as the free induction decay (FID). The FID decay rate is mainly determined by the spin dephasing caused by static magnetic field inhomogeneities. A process referred to as spin-echo measurement can substantially reestablish the spin decay in a non-homogeneous field.
Spin echo measurement can be described as in the following discussion. After a predetermined time period following the FID, another RF pulse is applied to the antenna. This RF pulse has predetermined amplitude and duration to realign the spin axes of the hydrogen nuclei in the earth formation by an axial rotation of 180 degrees from their immediately previous orientations, and is therefore referred to as a 180-degree pulse. After the end of the 180-degree pulse, hydrogen nuclear axes that were precessing at a slower rate are then positioned so that they are "ahead" of the faster precessing spin axes. The 180-degree reorientation of the nuclear spin axes therefore causes the faster precessing axes to be reoriented "behind" the slower precessing axes. The faster precessing axes then eventually "catch up" to, and come into approximate alignment with, the slower precessing axes after the 180-degree reorientation. As a large number of the spin axes thus become "rephased" with each other, the hydrogen nuclear axial precessions are again able to induce measurable voltages in the antenna. The voltages induced as a result of the rephasing of the hydrogen nuclear axes with each other after a 180-degree pulse are referred to as a "spin echo".
The spin echo induced voltage is typically smaller than the original voltage generated after cessation of the first RF pulse, because the aggregate nuclear axial alignment, and consequently the bulk magnetization, of the hydrogen nuclei at the time of the spin echo is at least partially realigned with the magnet's field and away from the sensitive axis of the antenna. The spin echo voltage itself decays by FID as the faster precessing nuclear axes quickly "dephase" from the slower precessing nuclear axes.
After another period of time, typically equal to two of the predetermined time periods between the initial 90-degree RF pulse and the 180-degree pulse, another RF pulse of substantially the same amplitude and duration as the 180-degree pulse is applied to the antenna. This subsequent RF pulse causes another 180-degree rotation of the spin axis orientation. This next 180-degree pulse, and the consequent spin axis realignment again causes the slower precessing spin axes to be positioned ahead of the faster precessing spin axes. Eventually another spin echo will occur and induce measurable voltages in the antenna. The induced voltages of this next spin echo will typically be smaller in amplitude than those of the previous spin echo.
Successive 180-degree RF pulses are applied to the antenna to generate successive spin echoes, each one typically having a smaller amplitude than the previous spin echo. The rate at which the peak amplitude of the spin echoes decays is related to petrophysical properties of interest of the earth formations. A certain number of spin echoes needed to determine the rate of spin echo amplitude decay is related to the properties of the earth formation; in some cases as many as 1,000 spin echoes may be needed to determine the amplitude decay corresponding to the properties of the earth formation which are of interest. The distribution of rates at which the peak amplitude of the spin echoes decay is directly related to parameters of interest in the earth formation.
As previously stated, NMR tools use an antenna for creating the RF field and for receiving the echo signal from the formation fluid being analyzed. An NMR antenna comprises typically a coil disposed around a core for increasing the inductance of the coil and to minimize eddy currents in the steel tool housing. The use of an intensifying core allows for a smaller antenna, which is particularly useful in downhole applications.
High-gain amplifiers are utilized to amplify low power echoes received prior to processing the signal. It is very important that the echo is distinguishable over noise. The ratio of signal amplitude to the noise amplitude, known as the signal to noise ratio, should be as high as possible. This will ensure that the echo can be distinguished even after amplification.
A primary source of noise known as ring-down is induced by mechanical oscillations within the antenna and other components of the sensor. A major cause of the oscillation is a certain characteristic of the antenna core material usually selected from a soft ferrite. Ferrite is a material that changes shape when in the presence of a magnetic field. The material then returns to the original shape when the magnetic field is removed. This property is known as magnetostriction. The ringing generated by magnetostriction is termed magnetostrictive ringing. From investigations of ferrite materials, it is known that the different materials exhibit different deformation characteristics when exposed to the same magnetic field. Some ferrites expand, while others contract with an applied field. Both types, as stated, return to the original dimensions when the field is removed.
One type of NMR well logging apparatus is described, for example in U.S. Pat. No. 4,350,955 issued to Jackson et al. The apparatus disclosed in the Jackson et al '955 patent includes permanent magnets configured to induce a magnetic field in the earth formations, which has a toroidal volume of substantially uniform magnetic field strength. This patent describes very well the physics of NMR technology and is hereby incorporated by reference.
An apparatus disclosed in U.K. patent application no, 2,141,236 filed by Clow et al and published on Dec. 12, 1984 provides improved signal-to-noise ratio when compared with the apparatus of Jackson et al '955 by including a high magnetic permeability ferrite in the antenna.
Another NMR well logging apparatus is described, for example in U.S. Pat. No. 4,710,713 issued to Taicher et al. The apparatus disclosed in the Taicher et al '713 patent includes a substantially cylindrical permanent magnet assembly which induces a static magnetic field having substantially uniform field strength within an annular cylindrical volume.
The apparatus disclosed in the Taicher et al '713 patent is especially subject to magnetoacoustic and magnetostrictive parasitic signals or "ringing". First, because the antenna is located within the strongest part of the magnet's field, when RF electrical pulses are applied to the antenna acoustic waves can be generated in the antenna by an effect known as the "Lorenz force". The antenna returns to its original shape in a series of damped mechanical oscillations in a process referred to as "magnetoacoustic ringing". Ringing can induce large voltages in the antenna which interfere with the measurement of the voltages induced by the NMR spin echoes. Additionally, the magnet is located in the highest strength portion of the RF magnetic field. The magnet can be deformed by magnetostriction. When each RF power pulse ends, the magnet tends to return to its original shape creating the magnetostrictive ringing as described above, which as magnetoacoustic ringing, can induce large voltages in the antenna making it difficult to measure the spin echoes.
Another NMR logging tool is described in U.S. Pat. No. 5,712,566 issued to Taicher et al. The '566 patent points out the drawbacks of the above referenced patents including the adverse effects of magnetoacoustic and magnetostrictive ringing. The '566 patent teaches a restrictive configuration approach to the problem of ringing. The approach is to first configure the permanent magnet as a cylinder having an axial bore. The antenna rod (the soft ferrite material most subject to magnetostrictive ringing) is placed within the bore of the permanent magnet. This specific configuration places the sensor antenna in the pole of the permanent magnetic thereby substantially reducing magnetoacoustic ringing. The particular placement of the antenna within the bore also reduces magnetostrictive ringing by allowing substantially complete demagnetization of the ferrite rod during the dead period of the RF signal. However, ringing will still occur because the RF field induces a magnetic field that encircles the ferrite rod.